As the move from analog communications to digital communications continues, several methods of modulation are being developed. One attractive choice is vector modulation where the resultant signal is the vector sum of two amplitude modulated signals. Each vector consists of a carrier signal modulated by a data input signal, where the two vectors are shifted in phase by predetermined amounts relative to one another. Particular examples of vector modulation are quaternary phase shift keying (QPSK), in which only phase shifts occur, and 64 QAM, in which both phase shifts and amplitude shifts occur. See Chapter 13 of Electronic Communications Systems by Wayne Tomasi, Prentice Hall, N.J., 1988, incorporated herein by reference, for a discussion of many of the vector modulation techniques. Two modulation input signals simultaneously modulate the in-phase component vector (I channel) and the quadrature component vector (Q channel), respectively, of the carrier signal. Usually the I and Q channels of the modulator are calibrated to have equal gain (equal output amplitudes for equivalent inputs) and to have an associated phase shift of precisely 90.degree.. This requires measurement and calibration of the vector modulator output signals to ensure equal gain in each channel, to ensure that the associated phase shift is precisely 90.degree. (or -90.degree.), and to remove any residual carrier signal present at the output when the input is zero (carrier leakage).
A vector modulator can be calibrated with a network analyzer that is connected to the carrier input terminals and to the modulated signal output terminals. The network analyzer can measure the amplitude and phase of the two output signals resulting from varying dc voltages applied separately to the I and Q modulation input terminals, and calibration can be performed. However, this method is expensive, cumbersome and often requires the use of additional equipment that cannot be incorporated into a vector modulator for self-calibration. This method is often limited in accuracy and may be subject to drift of some of the relevant parameters.
Edwards et al., in U.S. Pat. No. 4,717,894, discloses an iterative method of calibration of vector modulators using a scalar detector that measures the output signal issued from the modulator. An iterative four-step calibration process is followed and repeated until no further change in the results is observed. The quadrature phase error is minimized by adjusting two phase shifters provided with the vector modulator. The carrier leakage is then minimized by adjusting certain carrier leakage compensation sources to minimize RF output amplitudes with zero modulation signal applied to the input terminals of the modulator. The amplitudes of the I and Q modulation channels are then balanced by adjusting two signal attenuators associated with the data input terminals until the output amplitudes are equal for equal input amplitudes. Finally, the quadrature calibration sources are adjusted until the output amplitudes are balanced. This procedure must be repeated several times because each parameter to be calibrated cannot be accurately adjusted until the other parameters are all set perfectly.
Hedberg discloses apparatus for compensating for errors in a quadrature modulator in U.S. Pat. No. 4,890,301. Two quadrature signals, cos a(t) and sin a(t), are generated and mixed with two carrier signals, cos .omega.t and sin .omega.t, then added to form a standard sum signal cos [.omega.t-(t)]. Amplitude errors (A) and phase errors (V) in the sum signal are compensated for by incorporation of a first compensation network having three adjustable signal generators. Local oscillator leakage (L) is compensated for by a second compensation network, which includes two more adjustable signal generators whose output signals are subtracted from previously generated modulation signals. This approach appears to require iterative or repetitive choices of the adjustable compensation signals rather than providing optimal compensation in a single pass. No means is suggested by which the two compensation equations obtained by the inventor can be electronically implemented.
What is needed is method and apparatus for calibrating a vector modulator that does not require a network analyzer and does not require iteration so that each step of the procedure need only be performed once in order to calibrate the modulator.